1. Field of the Invention
The invention relates to photodetectors, and in particular, to a structure and method for producing high-speed, high-responsivity photodetectors.
2. Related Art
A photodetector converts an optical signal into an electrical signal. Photodetectors are therefore critical components in optical communications networks. FIG. 1 shows a conventional photodetector 100 that includes a PIN photoconversion structure 120 formed on a substrate 110. Electrodes 101 and 102 provide electrical connectivity for photodetector 100.
PIN photoconversion structure 120 includes a p-type anode layer 129, an intrinsic (undoped) absorption layer 125, an n-type cathode layer 121, and an etch stop layer 121-E. Absorption layer 125 absorbs light (photons) from an optical signal 190 that passes through anode layer 129. If the absorbed photons have sufficient energy (shorter wavelengths correspond to higher energy photons), electron-hole pairs are generated within absorption layer 125. The free electrons and holes move in opposite directions (electrons towards n-type cathode layer and holes towards p-type anode layer 129), thereby generating an electrical signal that can be correlated to optical signal 190.
The specific materials used in a photodetector depend on the wavelength(s) of light included in optical signal 190. For example, carrier wavelengths in modern optical networks are typically in the range of 1.3 to 1.55 μm. Therefore, telecommunications photodiodes typically include an absorption layer formed from indium gallium arsenide (InGaAs), which provides strong absorption characteristics in the critical 1.3–1.55 μm wavelength range.
In particular, conventional telecommunications photodetectors include an absorption layer 125 composed of In0.53Ga0.47As, as indicated in FIG. 1, which includes Group III sublattice concentrations (mole fractions) of 53% indium and 47% gallium. Note that the Group III sublattice of absorption layer 125 is made up of only the Group III elements (i.e., elements from column III of the Periodic Table) in absorption layer 125 (i.e., indium and gallium). In0.53Ga0.47As is used in conventional telecommunications photodetectors because it can be lattice-matched to an indium phosphide (InP) substrate 110, thereby ensuring the structural integrity of the photodetector.
For similar reasons, all the other components of photodetector 100 are also selected to have the same lattice constant as InP and In0.53Ga0.47As. For example, cathode layer 121 is an n-doped In0.52Al0.48As layer, which has the same lattice constant as In0.53Ga0.47As, anode layer 129 is a p-doped In0.53Ga0.47As layer, and etch stop layer 121-E is a thin InP layer. Etch stop layer 121-E is incorporated into PIN photoconversion structure 120 to simplify endpoint detection during the formation of cathode layer 121. Because etch processes are typically very selective, the InAlAs etch used to create cathode layer 121 essentially terminates at etch stop layer 121-E.
Reducing the vertical length (i.e., height or thickness) of absorption layer 125 in photodetector 100 generally increases the speed (bandwidth) of photodetector 100. The shortened absorption layer 125 means that charge carriers (i.e., the free electrons and holes) generated within absorption layer 125 in response to optical signal 190 have a shorter distance to travel to reach electrodes 101 and 102. This in turn means that photodetector 100 can respond to optical signals having higher modulation speeds.
In FIG. 1, the vertical length (i.e., thickness) of absorption layer 125 is indicated by length L1. The bitrate at which optical signal 190 can provide data to PIN photodetector 100 is therefore determined in large part by length L1. In general, reducing length L1 will increase the maximum bitrate (although at very small lengths L1, the capacitance of the device may limit further gains in bitrate). For example, conventional PIN photodetectors designed to support transmission rates (bitrates) of 40 Gb/s at a wavelength of 1.55 μm typically have an absorption layer 125 having a length (L1) of roughly 7000 Å.
Unfortunately, reducing the vertical length L1 of absorption layer 125 also reduces the responsivity of photodetector 100, since a thinner absorption layer absorbs less of incident optical signal 190 than a thick absorption layer. Therefore, as the vertical length of absorption layer 125 is reduced to improve detection speed, the detection efficiency, or responsivity, of photodetector 100 is reduced.
For example, if vertical length L1 is 1.4 μm, photodetector 100 will absorb roughly 61% of incoming optical signal 190. However, to support a 40 Gb/s transmission rate, absorption layer 125 in conventional photodetector 100 must be reduced to a vertical length L1 of 7000 Å. At this vertical length (thickness), absorption layer 125 will only absorb about 38% of incident optical signal 190. Consequently, conventional high-speed PIN photodetectors have difficulty detecting low-level optical signals.
To increase the responsivity of a PIN photodetector, a resonant-cavity approach can be used, in which reflective layers are formed that surround the absorption layer of the photodiode. When light is coupled into this resonant-cavity enhanced (RCE) photodetector, part of it is absorbed in the InGaAs absorption layer. The remainder passes down the detector until it is reflected back towards the absorption layer by the lower reflector. Some of this reflected light is absorbed in the absorption layer, while the remainder keeps traveling until it is reflected back towards the absorption layer by the upper reflector. This process continues until virtually all the light is absorbed.
Because an incoming optical signal passes through the absorption layer several times, an RCE photodetector with a thin absorption layer can still provide good responsivity. However, RCE photodetectors are often only useful for a very narrow band of wavelengths, since the semiconductor layers that are commonly used as the reflective layers are typcially only reflective over a narrow range of wavelengths. Furthermore, RCE photodetectors can be difficult to manufacture, due to the need for the additional reflective layers.
In an effort to overcome the limitations of conventional PIN photodetectors, other types of photoconversion structures are sometimes used. For example, in a waveguide PIN (WGPIN) structure, the optical signal is coupled into the edge of the detector (and therefore the edge of the absorption layer). Decreasing the vertical length (thickness) of the absorption layer in a WGPIN photodetector to improve bandwidth therefore does not significantly degrade responsivity, since the horizontal length of the absorption layer is not changed. Therefore, a WGPIN photodetector can provide both high responsivity and high bandwidth.
However, WGPIN photodetectors generally exhibit increased polarization dependent loss (PDL) as compared to other detectors, resulting in problematic signal distortion. Additionally, from a packaging standpoint, trying to successfully couple light from an optical fiber into a WGPIN detector can be difficult and time-consuming, which can signficantly raise the implementation cost of such photodetectors.
Accordingly, it is desirable to provide a method and structure for efficiently producing high-speed, high-responsivity photodetectors.